The present invention relates to a processing apparatus for processing objects to be processed, e.g., a vertical heat treatment apparatus.
Vertical heat treatment apparatuses, which can be cleaned, save space, etc., are generally used as treatment apparatuses for thermal diffusion steps and film forming steps of semiconductor device fabrication processes.
In such vertical heat treatment apparatus, a heat treatment furnace comprising a heater surrounding a cylindrical treatment vessel of quartz, a heat insulating material, etc. is vertically disposed in an upper part of the interior of a substantially rectangular casing. Below the heat treatment furnace in the casing there is provided a waiting space for a wafer boat with a number of semiconductor wafers or objects to be treated mounted thereon. The wafer boat stands by in the waiting space and is loaded or unloaded into or from the heat treatment furnace by lift means, e.g., a boat elevator.
By the use of such heat treatment apparatus, for example, various films, e.g., insulating films of SiO.sub.2 or others, are formed. In forming such films, the wafers are exposed to clean air in a wafer conveying system immediately before the film formation, and natural oxide (SiO.sub.2) films are adversely formed on the surfaces of the wafers. Characteristics of such natural oxide films are not good, as are characteristics of artificially formed oxide films by deposition or other techniques, and it is preferred to remove the natural oxide films as much as possible immediately before the films are formed. To this end a cleaning step of removing the natural oxide films formed on the surfaces of the wafers is incorporated. But the wafers are exposed to clean air after the cleaning on the way to the heat treatment apparatus for forming the films, and while standing by in the waiting space after being conveyed thereinto; thus, natural oxide films are again formed thereon.
Natural oxide films adversely formed by the start of the film formation are not serious in terms of device characteristics in a case of not so high integration. But in a case of high integration, as of 64M bits, or 256M bits, because the SiO.sub.2 films to be deposited in these situations are thinner, the natural oxide films affect device characteristics in a non-negligible manner. For example, when a total thickness of a SiO.sub.2 film including a 20 .ANG.-natural oxide film is 100 .ANG., a ratio occupied by the natural oxide film is insignificantly about 20%. But as described above, in a case of high integration, when a total thickness of a SiO.sub.2 film is a half of the above-described thickness, i.e., 50 .ANG., a ratio of the natural oxide film is as high as 40% which is non-negligible in terms of device characteristics.
Generally, the formation of natural oxide films depend on heat, water and oxygen concentrations.
A known structure for suppressing the generation of natural oxide films after cleaning wafers comprises the above-described waiting space for the wafers with the wafers that are standing by placed in an atmosphere of an inert gas, such as N.sub.2 (nitrogen) or others, and a load lock chamber in communication with the waiting space and placed in a vacuum or an atmosphere of an inert gas, such as N.sub.2 or others. In this way the wafers are kept as much as possible from contacting with clean air. A known lower level structure does not include the load lock chamber and includes only the waiting space, whereby clean air whose O.sub.2 (oxygen) is replaced with N.sub.2 to lower its O.sub.2 concentration to about 30 PPM, a natural oxide film suppressing gas, is circulated so as to suppress generation of the natural oxide films.
The above-described structure including the load lock chamber not only additionally requires the load lock chamber, but also has to use a large amount of expensive N.sub.2 gas. Thus higher equipment costs and running costs are necessary in order to greatly suppress the generation of natural oxide films.
The structure, which does not include the load lock chamber and which circulates the natural oxide film suppressing gas in the waiting space, does suppress the generation of natural oxide film and lowers equipment costs as compared with the structure that includes the load lock chamber. However, a large amount of expensive N.sub.2 gas is required under this technique so as to raise running costs, and this is a problem with this prior structure.
The present invention has been successfully made in view of this problem. An object of the present invention is to provide a processing apparatus which recirculates an inexpensive dry gas with a low dew point, whereby generation of natural oxide films can be suppressed.
With respect to suppressing the generation of natural oxide films, conventionally the oxygen concentration alone in a natural oxide film generation suppressing gas has been suppressed. The inventors have found that, based on results of their studies, the generation amount of natural oxide films does not depend on O.sub.2 concentrations when the water content in the natural oxide film generation suppressing gas is suppressed. By suppressing the water content the film generation can be suppressed to be small. Based on this finding, they have made the present invention.
To solve the above-described problem, the present invention relates to a processing apparatus comprising a processing vessel for processing objects to be processed; an objects mount for mounting the objects to be processed and being movable up and down into and out of the processing vessel through the bottom thereof; a waiting space provided below the processing vessel where the object mount stands by; and a natural oxide film generation suppressing gas supply system for supplying a natural oxide film generation the suppressing gas for suppressing generation of natural oxide films on the objects-to-be-processed in the waiting space, the natural oxide film generation suppressing gas being dried gas (air) with a low dew point.
In the processing apparatus of the present invention having the above-described constitution, the objects mount with objects to be processed is housed in the waiting space below the processing vessel, and the waiting space is supplied with a dry gas having a dew point by the natural oxide film generation suppressing system. Accordingly generation of natural oxide films on surfaces of the objects to be processed can be minimized.